In semiconductor fabrication, the metal process is normally required. Because metals contact patterned regions only, the photoresist process and the lithography method are mostly adopted currently for achieving the purpose of patternization. For normal semiconductor fabrication, which has higher costs, although the patterning method adds some extra costs, it is still relatively acceptable. Nonetheless, for low-cost solar-cell fabrication, using the photoresist process will apparently increase the cost. Thereby, the photoresist is not suitable for defining metal patterns in solar cells.
For the current metal process of solar cells, the screen printing method is adopted to form metal electrodes. The screen printing process is very simple and fast. Nonetheless, as crystalline silicon solar cells are developing towards the two directions of low costs and high efficiency, forming silver electrodes by screen printing faces bottlenecks. It is difficult to make breakthrough.
The cost difference between copper and silver is enormous while their conductivity values are approximate. Accordingly, under the consideration of costs, the International Technology Roadmap for Photovoltaic (ITRPV) predicts that the metal silver currently in use will be replaced by the metal copper gradually. The plated nickel/copper electrode is regarded as the potential choice of positive electrodes for the next-generation solar cells. In addition, the plating process enables electrodes to have better aspect ratios and thus the efficiency can be improved by reducing the shading area of solar cells. Besides, the metal contact resistance between nickel and silicon substrate is also superior to the silver paste in the screen printing process. Thereby, the efficiency of solar cell can be enhanced as well.
The current bottleneck of nickel/copper electrode plating process is that the patterning method for metals is too complicated or the equipment is costly. Thereby, the competitivity of nickel/copper plated electrodes cannot surpass that of the screen printing process. Take the laser ablation for example. It uses laser to burn and remove the existing anti-reflection coating (ARC) on the surface of solar cells. Afterwards, the metal plating procedure is performed. The ARC is an excellent blocking layer for plating. The region with the ARC removed can be plated with metal, and thereby achieving the purpose of metal patternization with ease. The laser process has the advantages of simplicity and rapidity, making it suitable for low-cost mass production.